Hydrogen fuel closer to reality because of storage advances

Advances made in rechargeable solid hydrogen fuel storage tanks.

March 21, 2012

How best to achieve the benchmark of 300 miles of travel without refueling? It may be to use the lightweight compound ammonia-borane to carry the hydrogen. With hydrogen accounting for almost 20 percent of its weight, this stable, non-flammable compound is one of the highest-capacity materials for storing hydrogen. In a car, the introduction of a chemical catalyst would release the hydrogen as needed, thus avoiding on-board storage of large quantities of flammable hydrogen gas. When the ammonia-borane fuel is depleted of hydrogen, it would be regenerated at a hydrogen station through a reverse reaction. From upper left to lower right, two sequential chemical reactions release two hydrogen molecules (green molecules under the hood) from two ammonia-borane molecules. The latter form a polymer, which can continue the hydrogen-releasing reaction. Illustration by Anthony Mancino.

An ammonia borane system could allow hydrogen to be easily extracted for use in hydrogen fuel cell batteries and then be recharged with hydrogen over and over.

Drive toward hydrogen vehicles just got shorter

A
significant advance in hydrogen storage could make hydrogen a more attractive
fuel for vehicles or other transportation modes.

Researchers
revealed the new single-stage method for recharging the hydrogen storage
compound ammonia borane. The LANL technology focuses on using ammonia borane as
a "chemical storage tank" for hydrogen fuel.

An ammonia
borane system could allow hydrogen to be easily extracted for use in hydrogen
fuel cell batteries and then be recharged with hydrogen over and over.

Vehicles could travel more than 300 miles on a tank

The
development could allow a vehicle to travel more than 300 miles on a single
"tank" of fuel, which is a benchmark for the U.S. Department of
Energy. The method represents a significant step toward practical use of
hydrogen in fuels.

In an
article in the March 18 edition of Science magazine, LANL and University of
Alabama researchers working within the U.S. Department of Energy’s Chemical
Hydrogen Storage Center of Excellence describe a significant advance in
hydrogen storage science.

Hydrogen as an ideal fuel but needs improved storage

Hydrogen is
in many ways an ideal fuel. It possesses a high energy content per unit mass
when compared to petroleum, and it can be used to run a fuel cell, which in
turn can be used to power a very clean engine.

On the down
side, H2 has a low energy content per unit volume versus petroleum (it is very
light and bulky). The crux of the hydrogen issue has been how to get enough of
the element on board a vehicle to power it a reasonable distance.

Ammonia borane boosts hydrogen storage capacity

Work at LANL
and elsewhere has focused on chemical hydrides for storing hydrogen, with one
material in particular, ammonia borane, taking center stage. Ammonia borane is
attractive because its hydrogen storage capacity approaches a whopping 20
percent by weight—enough that it should, with appropriate engineering, permit
hydrogen-fueled vehicles to go farther than 300 miles on a single
"tank," a benchmark set by the U.S. Department of Energy.

Hydrogen
release from ammonia borane has been well demonstrated, and its chief drawback
to use has been the lack of energy-efficient methods to reintroduce hydrogen
into the spent fuel once burned. In other words, until now, after hydrogen
release, the ammonia borane couldn’t be recycled efficiently enough.

Simple scheme reduces expense, complexity of the recycle stage

The Science
paper describes a simple scheme that regenerates ammonia borane from a hydrogen
depleted "spent fuel" form (called polyborazylene) back into usable
fuel via reactions taking place in a single container.

This “one
pot” method represents a significant step toward the practical use of hydrogen
in vehicles by potentially reducing the expense and complexity of the recycle
stage.

Regeneration
takes place in a sealed pressure vessel using hydrazine and liquid ammonia at
40 degrees Celsius and necessarily takes place off-board a vehicle. The
researchers envision vehicles with interchangeable hydrogen storage
"tanks" containing ammonia borane that are used, and sent back to a
factory for recharge.

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